DC Cabling
DC Wiring
If the battery is the heart of a marine electrical system, then cabling and connections are the arteries and veins, supplying the lifeblood to all the electrical loads. It’s no coincidence that most electrical failures on board can be traced to inadequate or corroded wires and connections, causing a loss of life blood, rather than a fault in the radio, chartplotter or other appliance.
To ensure reliable electrics, use correct wire and correct connectors. Using regular house wiring is totally inadequate. Premature failure will occur!
Although house wiring is made of copper, it is unsuitable for marine applications. Over time, copper will corrode, producing the familiar green patina. Effectively, the corrosion reduces the cross sectional area of the conductor which means that there will be an increased resistance to current flow. Voltage drops will occur, resulting in electrical system breakdowns and in extreme cases, overheating and even fire. House wire has fewer strands, making it less flexible and susceptible to fatigue failure due to vibrations.
The only cable to use in a marine environment is tinned, multi strand copper wire, preferably double insulated. Each strand of the cable is tin plated, which is actually electroplating with a nickel alloy, ensuring a high degree of corrosion resistance. Multi stranding ensures that the cable is flexible to withstand vibration. Copper is chosen for the conductor because of its low resistance to current flow.
| olor | Item | Use |
| Green | Bonding System | Bonding Wires (if insulated) |
| Yellow or Black | Ground | Return, Negative Mains |
| Red | Main Power Feeds | Positive Mains (particularly unfused) |
| Yellow w/ Red | Starting Circuit | Starting Switch to Solenoid |
| Yellow | Generator Field | Generator to Regulator Field Terminal |
| Brown w/ Yellow | Bilge Blowers | Fuse or Switch to Blower |
| Dark Gray | Navigation Lights | Fuse or Switch to Lights |
| Tachometer | Tachometer Sender to Gauge | |
| Brown | Generator Armature | Generator Armature to Regulator |
| Alternator Charge Light | Generator Terminal or Alternator Auxiliary Terminal to Regulator |
|
| Pumps | Fuse or Switch to Pumps | |
| Orange | Accessory Feed | Ammeter to Alternator or Generator Output and Accessory Fuses or Switches |
| Common Feed | Distribution Panel to Accessory Switch | |
| Purple | Ignition | Ignition Switch to Coil & Electrical Instruments |
| Instrument Feed | Distribution Panel Electrical Instruments | |
| Dark Blue | Cabin & Instrument | Fuse or Switch to Lights |
| Light Blue | Oil Pressure | Oil Pressure Sender to Gauge |
| Tan | Water Temperature | Water Temperature Sender to Gauge |
| Pink | Fuel Gauge | Fuel Gauge Sender to Gauge |
Cable Sizing
The purpose of choosing the correct cable size is to eliminate voltage drop. Voltage drop will always occur when there is resistance in a circuit. In cables, this is proportional to conductor material, cross sectional area and length. This can be calculated by formula or by voltage drop tables. Severely undersized cable will result in cables and connections heating up, causing failures and fires in extreme situations.
The voltage drop of a circuit must be measured when it is under full load. In practice, it is usually acceptable to tolerate up to a 10% voltage drop due to resistance in the cabling and connectors.
Unacceptable voltage drops do not only occur in high current situations such as starters and winches, but also in long runs of cable such as wiring to navigation lights up the mast. When installing voltage sensitive equipment such as electronics, cabling should be sized to limit voltage drop to 3%. This is why 3% and 10% loss tables are published. Also note that different tables apply for 12V and 24V circuits.
There are various conventions by which cable is sized. Cross sectional area is used in the scientific community and generally in Europe, arguably the best as it can be directly used in voltage drop calculations. Other standards are American Wire Gauge (AWG) and Battery & Starter (B&S) sizing, and auto electricians use a diameter based convention (e.g. 4mm or 6mm).
Right Size the Wire
To select the proper size wire to do a particular job requires you know two things.
First, you’ll need to know the the total distance between the power source (battery, bus bar, or circuit breaker) and the equipment being powered.
Next, you’ll need to know the current draw of the unit you are installing or rewiring. This is usually clearly stated in the accompanying documentation.
The table below shows wire sizes (purple) for low-voltage DC circuits (under 40 volts) based on the distance (red) from the power source to the appliance and the current draw (blue) of the unit. Wire sizes specified in the table should limit voltage drop in a 12-volt DC circuit to under 3%.
| Amps | 5 | 10 | 15 | 20 | 25 | 30 | 40 | 50 |
| 10 ft. | 18 | 14 | 12 | 12 | 10 | 10 | 8 | 8 |
| 15 ft. | 16 | 12 | 10 | 10 | 8 | 8 | 6 | 6 |
| 20 ft. | 14 | 12 | 10 | 8 | 8 | 6 | 6 | 4 |
| 25 ft. | 14 | 10 | 8 | 8 | 6 | 6 | 4 | 4 |
| 30 ft. | 12 | 10 | 8 | 6 | 6 | 4 | 4 | 2 |
| 40 ft. | 12 | 8 | 6 | 6 | 4 | 4 | 2 | 2 |
| 50 ft. | 10 | 8 | 6 | 4 | 4 | 2 | 2 | 1 |
| 60 ft. | 10 | 6 | 6 | 4 | 2 | 2 | 1 | 1/0 |
Cable Standards Comparison
The 60% duty cycle rating should, at best, be taken as a guideline. For heavier loads and long cable runs always use a calculation or voltage drop tables to determine cable size.
The perfect cable for wiring from the distribution panel to accessories such as lighting and electronics is 2mm² twin sheathed, tinned copper.
For larger loads, heavier grade cable is required. So for wiring from the battery to the distribution panel in small installations, 5mm² should be used. This cable is often used for larger bilge pumps.
10-16mm² cable is recommended for wiring from the battery to the panel on larger installations, as well as connections from the alternator to the battery.
For heavy loads such as starter motors, winches and electric outboards, even heavier grade cable is required. Starting and winch cable needs to be 32mm² to 90mm² and electric outboard cable should be at least 8mm². For loads such as these, it is essential that calculations or voltage drop tables be used to determine cable size.
*Kurt Küpper works with
Aquavolt Electric Boat Parts.
Tel: 02 9417 8455 www.aquavolt.com.au
DC Connections: TerminationsFor each length of cable in the electrical circuit, there are at least two connections required. Apart from fuel failures, electrical connections are arguably the primary culprit in boat breakdowns. Poor connections cause resistance to current flow, resulting in voltage drop. Each cable run must be correctly terminated so that the right voltage is present at the appliance, under load. There must be good contact between cable and terminations, requiring adequate contact area and pressure, free from corrosion, paint and other insulating materials. It is surprising how often a radio fault can be traced to a corroded connector which causes a voltage drop when transmitting.
This may generate some dissention, but I don’t believe in soldered joints on a boat. Primarily because soldered joints can create a brittle area which can eventually break due to vibration and which don’t spread the cable strands evenly in a screw connector. Also soldering can exacerbate corrosion. It is much easier and safer to use a crimp tool on board. So crimp connectors are the recommended method of terminating a cable. However, if you are a dedicated solderer, please continue to do so as I appreciate that there are two schools of thought on this subject.
Crimp Connectors
Every crimp connection is a potential trouble spot. The rule is to match the crimp connector to the cable and the equipment to which it is being terminated. Most connectors for light duty cable are made with insulated crimp connectors. The insulation of these is colour coded to indicate the cable size for which they are dimensioned:
Red conductor 0.5 to 1.65mm²
Blue conductor 1.04 to 2.63mm² (2mm² or 4mm auto cable)
Yellow conductor 2.63 to 6.64mm² (5mm² or 6mm auto cable)
Crimp ConnectorsThere are many types of crimp connectors including: eye (ring), fork, spade (aka quick connect or QC), piggy back, splices (inline butt splices or end splices), bullet, pin (wire or flat for use in screw down or crush type connectors). Special mention should be made that although fork connectors are easier to install than ring connectors, conversely they are more likely to fall off.
Your supplier should also be able to supply waterproof crimp connectors. These are usually butt splices which use heatshrink tubing as the insulator. Less well known are insulation displacement connectors which exude an insulating dielectric material.
Avoid using budget crimp tools and definitely don’t use pliers or a vice. The only proper tool to use is a ratchet crimper. It will not release the crimp until sufficient pressure has been applied, but it will not allow excessive pressure to be applied, which might damage the terminal. You will be glad you invested $70 for a good crimper, every time you use it.
As previously stated, it is essential to make sure that there is good electrical connection between the cable and terminal. Many amateurs have crimped on to insulation and wondered why current won’t flow through their beautiful crimp connection. Ensure that enough insulation has been stripped back from the cable to allow full contact between the conductor and the terminal and check that the tip of the conductor just protrudes from the front of the crimp barrel. After the job is done, test every crimp by giving the cable a decent tug.
Larger crimps for battery and winch cables require heavy duty crimp tools that are similar to swaging tools. They commonly have different dies for different cable and terminal sizes. It goes without saying that correct matching of cable, terminals and crimping dies, is essential for good connections. These tools are expensive and the average boat owner cannot justify their purchase. Ask your electrician or electrical supplier to crimp the terminals of your heavy duty cables or purchase pre-made cable assemblies (make sure that the cable is tinned!).
Finally, don’t be tempted to dispense with solder or crimp connectors and just wrap wires together or around a terminal post. There is no surer way to create the environment for a potential fire hazard.
Plugs & Sockets.
Plugs and Sockets
It’s difficult to get hold of a decent marine grade 12 volt plug and socket.The design brief is simple: The connection must be able to carry in excess of the required current without any voltage drop; it should be corrosion resistant and be easy to plug and unplug. Try to get something which satisfies these criteria at your local chandlery and you will probably be disappointed.
Typically, most manufacturers of 12V appliances still fit the anti-socially named cigarette lighter plugs. Some of these plugs are rated up to 16A but this is very optimistic.
Construction is generally inadequate for marine applications and corrosion resistance is usually woeful. Most are designed to have a push fit interface. Because this relies on pressure only for the electrical contact, vibration will invariably result in interruption to the current flow. In particular, if you have a portable fridge equipped with a traditional lighter socket, you should consider upgrading to a more robust interface.
A better bet is the lighter-style termed DIN type. They are more compact, have better quality components and have a more positive locking mechanism. They are also rated up to 16A at 12V. Some also have a removable ring which enables the plug to also be fitted to the common lighter socket.
Another common variety of connector is the chromed brass fitting. They are commonly available with a locking ring and a chained socket cap when the lug is not in use. Theoretically this should make them ideal for deck mounting where water ingress could be a problem. They are available in both two and four pin varieties. While they potentially could have been good connectors, most of the products on the market are of poor quality.
Dimensional variations of components purchased at different times have been known to be incompatible. Also the brass screws securing the cables have a very fine thread and screw head which is easily stripped, resulting in a poor connection.
More positive connections can be made using 32V15A connectors which are similar to the type used in 240Vac systems, that is, the common domestic power plug. Instead of three pins, there are only two in a T-configuration. These form excellent electrical connections although they are not of very elegant appearance.
An interesting new generation development is the Magcode connector. The port (socket) has surface terminals and no holes. A magnet in the clip (plug) secures the connection and activates a switch in the port, ie: the terminals in the port only become live once the clip is in position. 12V and 24V components have opposite magnetic polarity, ie: a 12 volt clip cannot be connected to a 24V port in error. Also available as a twist-on positive lock version in cases where tension may be encountered on the lead. Rated to 25A and highly recommended, especially in dual voltage installations.
Waterproof plastic connectors such as Bulgin or Amphenol are very high quality with high ingress protection ratings to ensure long service in a marine environment. These are available with multiple contacts and socket mountings such as inline, flush mount and surface mount. They are really durable plugs and sockets which are not cheap but worth the cost in the long run.
When it comes to connecting high current circuits such as trolling motors and winches, there is no substitute for the Anderson connector. They are high current connectors, rated 50A, 175A or 350A. They are unsexed, meaning that two identical components are used for a connection and are available only in two pole format. It should be noted that they are not waterproof. Nowadays, lower current connectors based on the Anderson design are available. These are single pole connectors which can be clipped together to form multiple connectors.
Obviously there are many other varieties of connectors, such as plugs and sockets designed by Marinco for use with trolling motors. These are good quality units which can be used for other requirements where higher currents up to 50A at 12V, are required. They have a three pin twist lock interface.
The bottom line is that you definitely get what you pay for in terms of plug and socket reliability.
*Gavin Sorrell works in collaboration with Aquavolt Electric Boat Parts.
Tel: 02 9417 8455 www.aquavolt.com.au